An important aspect of the manufacture of filter assemblies is the process of bonding an end cap to a filter pack. As used herein, the term "filter pack" refers to any structure that includes a filter medium. The filter medium may be, but is not limited to, a metal mesh or a porous metal medium formed from metal powder or metal fibers.
One conventional method of bonding is known as brazing. The end of a filter pack is placed in an end cap with a brazing alloy and the resultant assembly heated until the brazing alloy melts. The assembly is then cooled and as the molten brazing alloy solidifies, it bonds the end cap to the filter pack.
One disadvantage of this brazing method is that the molten brazing alloy may wick up into or along the filter pack for a significant distance away from the end cap. This sometimes starves the joint between the filter pack and the end cap for brazing alloy, resulting in incomplete bonding. In addition, when the molten brazing alloy solidifies, it then blinds the porous filter medium, i.e., closes off the pores of the filter medium to fluid flow. This significantly decreases the effective filtration area of the filter medium and reduces the efficiency of the filter assembly.
In another conventional method of bonding, the end of a filter pack is compressed to form a dense area. A stainless steel ring and the compressed end of the filter are then positioned in an end cap. The entire assembly is then welded together.
This method gives rise to several disadvantages. By compressing the ends of the filter pack, the effective filtration area is reduced, thereby reducing the efficiency of the filter assembly. Compressing the ends of the filter pack may also damage the filter medium, creating leaks which are difficult to repair. This is a problem especially associated with pleated filter assemblies having wide, open pleats. Special tooling is required, which renders this method undesirably expensive. Further, welding the filter pack to the end cap frequently results in discoloration of the filter assembly.